An update to as-yet unused new feature of snprintf, which was added to bring

our sprintf()-alike in sync with our fprintf()-alike features.


strings/my_vsnprintf.c:
  Advance the destination pointer properly.
  
  Also, pay attention to the "n" in snprintf() -- never write too much.

strings/my_vsnprintf.c

@@ -99,7 +99,11 @@ int my_vsnprintf(char *to, size_t n, const char* fmt, va_list ap)
     else if (*fmt == 'b')				/* Buffer parameter */
     {
       char *par = va_arg(ap, char *);
-      to=memmove(to, par, abs(width));
+      DBUG_ASSERT(to <= end);
+      if (to + abs(width) + 1 > end)
+        width= end - to - 1;  /* sign doesn't matter */
+      memmove(to, par, abs(width));
+      to+= width;
       continue;
     }
     else if (*fmt == 'd' || *fmt == 'u'|| *fmt== 'x')	/* Integer parameter */